and a 100%, it's like building a brick wall around a city and protecting it against an enemy. it's excellent protection against something entering our community that could cause illness. >> narrator: very strong herd community could completely eradicate many vaccine- preventable diseases, rendering vaccines unnecessary. this happened with smallpox in the 1970s. a decade ago, the world health organization had plans to eradicate polio, followed by measles, but vaccine scares around the world served to chip away at herd immunity, pushing these goals into the future. >> many of these germs belong to us only, which means if we can make enough humans immune simultaneously, we could eradicate them. and then, yes, we could stop vaccinating. there was actually a campaign hoping to eradicate polio from planet earth, as we did small pox, and notice we're not vaccinating anymore for smallpox because it's gone. but at the moment, at least, while they're in the world, there still is the potential for outbreaks if people are willing to stop vaccinating prematurely. >> narrator: why is it so